Wire section and method of dewatering a fiber web in a wire section web

Abstract

The invention concerns the wire section of a papermaking machine and a method of removing water from a fiber web in that section. A fiber suspension is discharged from a headbox on at least one and usually between two wires in the forming section. The fiber suspension is dewatered until it eventually reaches the point of immobility at which the fiber orientation is fixed. The rate of removal of water transverse to the direction of travel of the web is controlled at regions across the web. In particular, the rate of water removal is decreased at the edge regions of the wire and the web, as compared with the center region, to improve the transverse formation profile of the web. Various techniques for controlling the rate of water removal are disclosed including using different mesh of the wire at the edge regions and at the center region, a suction forming roll applying different levels of suction at different zones across the wire and the web, and adjustable ledges engaging one of the two wires and being adapted or adjusted to apply different pressure levels to the wire across the web.

What is claimed is: 
     
       1. A method of removing water from a fiber suspension and for controlling a transverse profile of the fibers in the suspension in a forming section of a paper machine, the fibers within the suspension being capable of changing their orientations until the suspension is dewatered to a point of immobility when the orientation of the fibers do not change, the method comprising the steps of: supplying the fiber suspension onto at least one wire of the forming section of the paper machine;   exposing the fiber suspension of the web to: (i) a level of suction at its center; and (ii) respective levels of suction at its lateral edges in a manner for dewatering the fiber suspension through the wire; and   controlling the rate of removal of water from the fiber suspension at edges of the web as compared with a center region of the web prior to the point of immobility of the fibers in the suspension by providing a different level of suction at the edge regions than at the center region of the web.   
     
     
       2. The method of claim 1, wherein controlling the rate of removal of water from the web comprises removing water from the web at a greater rate at the center region of the web than at the lateral edge regions. 
     
     
       3. The method of claim 1, wherein controlling the rate of removal of water comprises applying suction to the fiber suspension on the wire and controlling the levels of suction applied at zones across the wire and the web. 
     
     
       4. The method of claim 3, wherein the rate of water removal is controlled by applying lower levels of suction at zones at the edges of the web than at zones at the center of the web across the web on the wire. 
     
     
       5. The method of claim 1, wherein the forming section includes two wires traveling one above the other and the suspension is supplied between the two wires of the forming section and travels through the forming section between the two wires. 
     
     
       6. The method of claim 1, further comprising spraying the suspension onto the wire through the outlet of a headbox. 
     
     
       7. A method of removing water from a fiber suspension and for controlling a transverse profile of the fiber in the suspension in a forming section of a paper machine, the fibers within the suspension being capable of changing their orientations until the suspension is dewatered to a point of immobility when the orientation fibers do not change, the method comprising supplying the fiber suspension between at least two wires in the forming section of the paper machine, the wires having different resistances to the passage of water therethrough in edge regions of the wires than at a center region of the wires to control the rate of removal of water from the fiber suspension at edges of the web as compared with a center region of the web prior to the point of immobility of the fibers in the suspension. 
     
     
       8. A forming section of a paper making machine receiving a fiber suspension from a headbox, the fibers within the suspension being capable of changing their orientations until the suspension is dewatered to a point of immobility when the orientation of the fibers do not change, the forming section comprising: at least one wire operable to receive the fiber suspension from the headbox, the wire having different resistances to the passage of water therethrough in edge regions of the wire than at a center region of the wire such that the rate of removal of water from the fiber suspension is controlled at edges of the web as compared with a center region of the web prior to the point of immobility of the fibers in the suspension.   
     
     
       9. The wire section of claim 8, wherein the wire comprises a porous mesh and the wire has a respective different size mesh in the edge regions thereof and in the center region thereof across the wire for achieving different resistances to removal of water through the wire mesh. 
     
     
       10. A forming section of a paper making machine for receiving a fiber suspension to be dewatered, the fibers within the suspension being capable of changing their orientations until the suspension is dewatered to a point of immobility when the orientation of the fibers do not change, the forming section including: at least one forming section wire which receives the fiber suspension dispensed from the headbox, moves the wire through the forming section, and permits dewatering of the fiber suspension through the wire as the wire moves;   a suction apparatus which controls a rate of removal of water from the side edge regions of the fiber suspension of the web as compared with a rate of removal of water from the center region of the fiber suspension of the web across the wire such that respective selectable levels of suction at different zones across the wire are obtained prior to the point of immobility.   
     
     
       11. The forming section of claim 10, wherein the suction apparatus comprises a forming roll about which the wire is partially wrapped, the forming roll including means therein for selectively applying respective levels of suction at different regions across the wire and along the axis of the forming roll. 
     
     
       12. The forming section of claim 11, wherein the suction roll is divided into at least two lateral edge zones toward the edge regions of the wire and a central zone between the edge zones, and at each of the zones of the suction roll, the suction roll is selectively operable for applying respective levels of suction. 
     
     
       13. The forming section of claim 12, wherein the lateral widths of the zones of the forming roll are variable across the wire. 
     
     
       14. The forming section of claim 12, further comprising partitions in the suction roll for separating adjacent zones within the suction roll, and the partitions being displaceable in the axial direction along the suction roll for altering the axial length of each of the zones. 
     
     
       15. The forming section of claim 12, further comprising means for applying selective different levels of suction to the respective zones of the suction roll. 
     
     
       16. The forming section of claim 15, further comprising partitions in the suction roll for selectively varying the circumferential length around the suction roll over which suction can be applied of each zone along the roll, each suction zone having a respective circumferential length around the forming roll. 
     
     
       17. The forming section of claim 15, wherein the vacuum levels in at least the lateral edge zones of the forming roll are adjustable. 
     
     
       18. The forming section of claim 15, wherein the forming roll has a roll shell which is porous and the zones within the forming roll communicate through the porous shell of the forming roll, the porous shell being formed to have different resistances to flow of water through the roll shell over the axial length of the roll for selectively controlling the rate of removal of water from the web on the wire passing over the roll shell.